UPDATE, l was wrong , the red and white telehandler , is the same it wont pick up on flat level ground , and when it does , it hasn't got a strong grip . it starts vibrating , then drops it .
So when all the telehandlers do the same when the log attachment is used , l,m thinking the log attachment is not programed properly.
Needs a strong grip, and when grabbing a log from above , the grip goes right thru the log and don't pick up , if you try to pick up by leaving the 2 bottom lifters flat ,then the top part of the lifter does the same , after an hr of messing with it, you may be able to pick up a log.

Forestry is far from perfect in the game and is also probably the feature where you need to practice the most to be able to do it.
But after hours and hours of practice you learn how to go about it to handle/adapt to the short comings of the game engine.
Ones you get a "grip of it" it is well worth the time spent on building your skill level as it is quite a lot of fun!

This is one of the features of the game that can not be done in a "rush" ...
Easy on the sticks and a lot of patience is the trick!

When it comes to picking up logs with the telehandler log fork I pick it up from "above" ... and also if you have problem with the logs "vibrating" try loosen the grip a bit and it will be more "relaxed".
As the grip has "issues", the best practice is to pick up the logs so that they are well balanced in the fork (a must if you want to be able to loosen the grip).

Non of the forestry equipment works any good , thinking about selling them all back , and stick with farming.
l took a huge load of logs on the big trailer to the train , got out to operate the crane , got lined up above the logs , lowerd the claw , tightened it up , lifted up the logs , BOOM , logs flying everywhere , what a nightmare complete ******** haaaaaaaa , ended up taking logs one by one to the wood selling place on the ground by the shacks ,and even that was no picnic , telehandler kept dropping every one wheather you grabbed it from above or try to slide it under the log , bludy joke , and the whole huge trailer load of huge logs only paid me 20,000 , l can make more then that on a 5 min mission harvesting wheat.
l spent 2 days harvesting them logs , not worth it